1. Field of the Invention
The present invention relates generally to atom phase-controlled double rephasing-based quantum memory and, more particularly, to a modified photon echo method using a nonlinear optical medium having three energy levels and at least five optical pulses oscillating among the energy levels of the optical medium.
2. Description of the Related Art
In an information processing device such as a computer, a memory is one of the key elements of logic circuits. Unlike conventional optical memory currently commercialized, a quantum memory should store a quantum state and reproduce the stored quantum state in an arbitrary time. Quantum memory is required to have fidelity higher than 67%.
Furthermore, in order to perform long-distance quantum communications, a quantum repeater capable of restoring the attenuation of quantum signals is required. The key component of this quantum repeater is quantum memory. The requirement of the quantum memory is a long storage time equal to or longer than one second. However, the longest storage time of quantum memory hitherto known is merely tens of milliseconds.
Meanwhile, a photon echo is a method using the different phases of atomic coherence generated via light interactions with a resonant medium having two energy levels. A photon echo uses reversible inhomogeneous broadening, where each phase of atomic coherence broadly distributed along the inhomogeneous broadening is swapped by an optical n-pulse that allows population swapping between the two energy levels.
In this case, inhomogeneously broadened atoms have a random spectral detuning against the resonating light. The phase evolution speed of each excited atom is proportional to the detuning determined by the spectral distribution. Thus, the sum of entire atomic phases quickly disappears, where the dephasing time is inversely proportional to spectral width of atomic distribution.
When a π-pulse is applied and the medium's population is swapped, the coherence evolution direction is reversed and initial phase is recovered at last. This recovered atomic coherence is referred to as a photon echo. The photon echo and the medium's population inversion have an inseparable relation.
However, a photon echo inevitably accompanies spontaneous emission process as well as stimulated emission under the population inversion by the π pulse.
Furthermore, in conventional optical information processing, a stimulated emission phenomenon acts as signal gain. In contrast, quantum information processing, spontaneous emission or stimulated emission acts as noise according to the no-cloning theorem. Thus, a photon echo scheme cannot be fundamentally applied to quantum memory.
Conventional quantum memory schemes were filed for patents and published in a plurality of documents including Korean Unexamined Patent Application Publication No. 10-2010-0016999 (hereinafter referred to as “prior art document”).
The quantum memory schemes disclosed in the prior art documents include quantum memory that includes first and second ground states that are proximate in terms of energy or are degenerate and prevent dipole transition, an excited state that allows two-photon transition between the first and second ground states, allows spin coherence and has spin inhomogeneous characteristics, and a proximate ground state that prevents dipole transition in connection with the first and second ground states and allows only transition in connection with the excited state, performs two-photon transition from the first and second ground states to the excited state and induces spin coherence, and then transitions to the proximate ground state and freezes the phase of the spin coherence, thereby storing data.
The conventional technology disclosed in the above-described prior art document is configured to store optical data in spins using Raman coherence based on spin inhomogeneous broadening, unlike photon echo-based quantum memory using optical transition inhomogeneous broadening. Although the conventional technology can lengthen the storage time up to several hours determined by the spin population decay time, it has not solved the population inversion problem offering quantum noises.